This invention relates to a technique for recovering data and clock signals from an encoded serial data stream and more particularly relates to a decoder for decoding bi-phase and Manchester coding.
Bi-phase encoding of serial data for transmission over wire or fiber optic lines is commonly used because the code has a 50% duty cycle and is self-clocking. Bi-phase encoding is also known as Manchester, phase encoding, and frequency modulation. There are three versions of bi-phase encoding designated "l", "m" and "s".
The "l" or level version has transitions in the middle of the bit period and uses rising edges for a zero and falling edges for a one. The "m" or mark version has transitions at the beginning of every bit period and also has a transition at the middle of a one bit. The "s" or space version has transitions at the beginning of every bit period and also has a transition at the middle of a zero bit. The "l" version is usually called Manchester encoding.
The standard way of decoding bi-phase data is to use a Phase Locked Loop (PLL) to recover the clock and then to use the clock to recover the original NRZ (non return to zero) bit stream. Although the PLL is capable of operating at 100 megabits per second, the PLL must lock in phase with the clock signal derived from the demodulation, and there is a certain amount of settling time required for the PLL to lock in. Thus the demodulator clocked by a PLL would require a definite period of "training" or preamble during which the transmitted encoded signal contains only reference edges. For bi-phase this period would be a sequence of consecutive zeros to be encoded; for Manchester the sequence would be one, zero, one, zero, etc. In either case the encoded waveform for training is a series of alternately high and low levels with each having a full bit interval duration. For situations requiring transmission of bursty data at very high data rates, a preamble is required at the beginning of each burst and thus limits the data throughput. It is desirable to have a decoder that uses little or no preamble to increase data throughput.
A decoder that uses little or no preamble is described in Hewlett Packard's Opto-Electronics/Fiber Optics Applications Manual, Second Edition, at page 10.16. The Hewlett Packard demodulator does not require long training; it requires only one full bit interval with no edges followed by a reference edge (in bi-phase, a single encoded zero; in Manchester, a one, zero or zero, one). The reason for the rapid training is that a pair of monostable vibrators (one shots) are used. While this decoder requires little or no preamble, the problem with this encoder is that one shots are limited in speed of operation and thus the entire decoder is limited to about 10 to 20 megabits per second.
Therefore it is an object of the present invention to provide a bi-phase decoder that can decode bursty type data without lengthy preamble.
It is another object of the present invention to provide a bi-phase decoder which can decode data at the rate of 100 megabits per second or greater.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.